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We built the subfloor in our van conversion for stability and longevity to “stand” the test of time. Down the road, if we want to change our layout and refresh the van, we want to just re-arrange some cabinets. We don’t want to make any changes to the subfloor!
Our components are simple:
Insulating the floor is important, but not as important as insulating our walls & ceiling. The warmth inside our van will be rising. Every inch of insulation we add is an inch of headroom we lose in the van, so we chose 2” of insulation in the ceiling, and 1” in the flooring.
We used polyiso insulation because it has the highest R Value per inch (6.0) of any rigid insulation, and it has high compressive strength (16-25 pounds per square inch, or psi). This means we and all our friends can (and will!) stand and jump on our floor without fear of it collapsing.
Some builders screw or bolt their joists into the van metal. We did not consider it necessary. The subfloor is secured to the van by gluing the joists down to the van metal using Sikaflex 221.
- Cabinets and interior elements are bolted to van walls, not the floor. In a car accident, these hold our cabinets and everything in place. The subfloor is for standing on and that’s it. It doesn’t provide structural integrity for any of our build
- High strength glue will last many years. Gravity is on its side! If glue fails, joists still won’t move as they are each screwed to plywood, which is butted up to walls and adjacent plywood
- Screwing through the van metal floor was undesirable as it would create dozens of potential rust/leak spots
Awkward Van Spot
The van metal floor has ribs. These are almost 0.5” tall, but not quite! Don’t try to fill the low spots with something 0.5” tall, because those will be slightly proud of the ribs (we speak from experience)!
In the end, we placed 1.5” tall lumber in the low spots as joists, then 1” polyiso sitting on the ribs. The specific lumber was 2” x 3” (actual measurement 1.5” x 2.5”).
How far apart should the joists be, both side-to-side and front-to-back? We can say that our subfloor feels super sturdy! Side-to-side spacing between our joists is 18 inches on center at its greatest. The span rating for 1/2 inch plywood is 32/16, or 32” between joists when plywood is sheathing for a roof (not much expected weight on it), and 16 inches as a floor (lots of expected weight on it). In standard construction, subfloors are ¾ inch plywood or thicker. But we want as thin as reasonable. Benefits of our 1/2″ plywood include lighter weight, lower cost, and an extra ¼ inch of headroom!
We feel comfortable going slightly wider than the recommended subfloor spacing since we won’t have that much weight. And if the plywood buckles slightly, the high compressive strength polyiso is sitting just about an eighth of an inch underneath our joists.
Span ratings for standard sheathing plywood are printed in big black letters on the plywood. Take note these are for normal buildings, and we don’t intend to subject our van to the same stresses a commercial building undergoes. For reference, per the APA – The Engineered Wood Association (exciting group!), the spans for some common plywood thicknesses are (roof sheathing span / subfloor span):
- 5/16”: 20/0
- ⅜”: 24/0
- ½”: 32/16
- ⅝” 40/20
- ¾”: 48/24
The edges of the plywood sheets are their weakest spot. We used four 4×8 sheets of plywood.
- A joist is necessary underneath all joints/edges of plywood. Therefore, we measured and planned where the edges would land, and ensured a joist would bridge the gap between the plywood sheets.
- Our main entryway at the van sliding door will endure a lot of traffic and it needs support. However, there is a metal rib here, and recall we have been placing our joists on the low spots of the metal floor, not on the ribs. Therefore, we used a 2”x4” here (wider so that it spanned the width of the rib), and notched the underside 0.5” depth with a table saw. Now the 2”x4” is sitting on the van metal floor. When screwing the plywood down to this joist though, we needed to remember to use shorter 1” screws here (we used 1.5” for all other joists), since the depth of the wood is less after being notched.
Awkward Van Spot
The back one foot or so of the van metal floor is not flat and curves up (and down) ever so slightly. This was not significant enough for any drastic alterations, and our solution was:
- Make the back joists very short (4” or so) so that the shortest possible material was subjected to unlevel metal
- Glob lots of adhesive on the portions of the joists that wouldn’t be flush with metal, to “bridge the gap” from the wood to metal
- Use a plywood that was slightly bowed, and cut and place it so that the already-present upward bow of the plywood was relatively in line with the upward curve of the joists
Simple – we put adhesive on the lumber, set them on the metal, and put some weight on top of them! We made a rookie mistake though, which was initially using standard Liquid Nails adhesive because we had some on hand.
We need to read the label first, and make sure the adhesive is compatible with metal. Metal needs a stronger adhesive than other materials since it is not porous.
Lots of adhesive is used in a van build, and the best choice is Sikaflex-221. Insanely strong on metal, water resistant, non-corrosive, lower-VOC, and flexible (it’s in the name) which is important in a vehicle.
For maximum coverage and strength, we use full 4’x8’ sheets of polyiso insulation, and cut out spaces for the joists. There’s a simple trick to this!
- Cut polyiso to size, including wheel well area and notches for various vertical ribs
- Paint the joists
- Carefully set the polyiso on top of the wet joists
- Voila, flip your polyiso over and just cut out the parts where the wet paint is marked!
We ran our polyiso perpendicular to the plywood (polyiso is front to back, plywood is side to side) to maximize stability in the final floor.
The easiest way to cut polyiso insulation is to heat up a butter knife until red hot (we used a propane torch), then it cuts like butter. A saw will simply tear the insulation which is much less pleasant or precise.
Safety note, cutting polyiso with a hot knife produces vapor/fumes that smell funny and we assume are unhealthy to inhale. In our picture Nadia is wearing a dust mask which in reality does very little to stop these fumes because the dust mask is too porous. We couldn’t be bothered to put on a proper respirator. Good news though, neither of us developed any cancers yet!
After we cut and placed all the polyiso, we placed aluminum foil tape over all the joists to complete the vapor barrier in the floor. We ended up abandoning the plan for a vapor barrier for the rest of the van, and in the end we don’t believe this step is necessary.
Awkward Van Spot
The spare tire is under the van, and is lowered by turning a bolt housed in a compartment with a black cover (lug wrench and jack handle are in compartment below passenger seat). We wanted to bring our flooring all the way to the back of the van. Therefore, we need to keep this compartment on the metal floor accessible!
Using a 2″ holesaw, we made a hole in our subfloor and final floor to keep this accessible.
We cut the 4×8 sheets of ½ inch plywood to size using a circular saw. Importantly, we left approximately 1/16″ inch of space between each plywood sheet, as well as between the plywood and the van walls. These are known as expansion gaps and are necessary as the plywood will naturally expand and contract. If there is no gap, then the plywood will bend and buckle when it expands.
Next came templates for the wheel wells, and we measured the notches for any other necessary cut outs. Fortunately these do not need to be perfect, as 1) the gaps will be filled with expanding foam, 2) a flooring material will be on top of all the plywood anyways, and 3) wheel wells won’t be visible from the living area!
Now comes the fun part, fastening the plywood! We used 1.5” screws that could countersink (any screw that has a cone head instead of a flat head). We started with the front plywood panel, measured from the front and the walls where the joists were, then transferred those measurements to the plywood and marked where screws should be placed. For the other three plywood panels, the process was easier:
To complete the floor and the vapor barrier, we sprayed small amounts of expanding spray foam along most of the edges of the plywood, where it meets the van walls. This was carved down to be flush with the plywood.
When you’re done with your comfortable subfloor, enjoy macarons and sparkling white grape juice with friends!
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